Strong vs Weak Acids (Higher Tier)

Definition of Strong and Weak Acids

Acids are substances that release hydrogen ions (H⁺) when dissolved in water. The strength of an acid depends on how completely it ionises in aqueous solution.

• Strong acids fully ionise in water, meaning all acid molecules split to release H⁺ ions.
• Weak acids only partially ionise in water, so only some acid molecules release H⁺ ions.

Examples of strong acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃).

Examples of weak acids include ethanoic acid (CH₃COOH), citric acid, and carbonic acid (H₂CO₃).

Ionisation in Aqueous Solution

When acids dissolve in water, they release hydrogen ions (H⁺) which cause the acidic properties.

• Strong acids fully ionise, so the concentration of H⁺ ions in solution is high.
• Weak acids partially ionise, so the concentration of H⁺ ions is lower compared to a strong acid of the same concentration.

For weak acids, ionisation is reversible and establishes an equilibrium between undissociated acid molecules and ions:

HA (aq) 77 H⁺ (aq) + A^2 (aq)

Here, HA is the weak acid molecule, and A⁻ is its conjugate base.

Because only some molecules ionise, the solution contains both ions and undissociated acid molecules.

This difference in ionisation explains why strong acids produce more H⁺ ions and weak acids produce fewer.

pH and Acid Strength

The pH scale measures how acidic or alkaline a solution is, ranging from 0 to 14.

• Lower pH values mean higher acidity and stronger acids.
• Strong acids typically have pH values between 1 and 3.
• Weak acids have higher pH values, usually between 3 and 6, depending on concentration.

Indicators such as methyl orange or universal indicator can be used to estimate pH by colour change. pH probes provide a more precise digital measurement.

For example, a 0.1 mol/dm³ hydrochloric acid solution (strong acid) has a pH around 1, while a 0.1 mol/dm³ ethanoic acid solution (weak acid) has a pH around 3.

For instance, if you measure the pH of 0.01 mol/dm³ hydrochloric acid, the pH is about 2 because it fully ionises:

Concentration of H⁺ ions = 0.01 mol/dm³

pH = $-\log_{10}(0.01) = 2$

Reactivity Differences

Strong acids react faster with metals than weak acids because they release more H⁺ ions, which react with the metal.

• More H⁺ ions mean more frequent successful collisions with metal atoms, increasing the rate of reaction.
• Weak acids react slower because fewer H⁺ ions are available.

This difference is important in industry and laboratories where reaction speed matters. For example, hydrochloric acid is often used when a fast reaction is needed, while ethanoic acid might be used when a slower, gentler reaction is preferred.

For example, when magnesium reacts with hydrochloric acid, bubbles of hydrogen gas form quickly:

Mg (s) + 2HCl (aq) 7 MgCl₂ (aq) + H₂ (g)

With ethanoic acid, the same reaction occurs but much more slowly.

Inline example: Calculate the pH of a 0.02 mol/dm³ hydrochloric acid solution.

Since HCl fully ionises, [H⁺] = 0.02 mol/dm³. Therefore, $\mathrm{pH} = -\log_{10}(0.02) \approx 1.7$.